1. Field of the Invention
This invention relates to electrostatic powder painting, and more particularly to improved tribo-electric powder spray guns.
2. Description of the Prior Art
In electrostatic powder painting, dry paint particles are fluidized in a powder hopper and pumped through a hose to a spray gun which sprays the powder onto a product to be coated. The spray gun typically charges the powder in one of two ways--either the gun has a high voltage charging electrode, or the gun has means to charge the powder by friction, i.e., tribo-electrically. This invention relates to tribo-electric powder spray guns.
Generally, in tribo-electric powder guns, the powder is epoxy based, and surfaces are provided within the gun, typically constructed from polytetrafluoroethylene (PTFE), which the powder particles impact numerous times to frictionally charge the particles. When the powder particles are sprayed from the front of the gun, they are electrostatically attracted to the product to be painted which is generally electrically grounded and suspended from an overhead conveyer. Once these electrostatically charged powder particles are deposited onto the product, they adhere there by electrostatic attraction until they are conveyed into an oven where they are melted to flow together to form a continuous coating on the product. Powder coating generally provides a tough and durable finish such as would be found on many appliances, garden furniture, lawn mowers, and other products.
One commercially available tribo-electric powder spray gun is shown in U.S. Pat. No. 4,399,945. This gun is available as a Tribomatic.RTM. gun from Nordson Corporation, Amherst, Ohio. In this gun, the powder is charged in a bundle of curved PTFE tubes which are wrapped around a core. As the powder passes through the tubes, it impacts the interior walls of the tubes several times and picks up charge upon each contact. The outer layer of the tube bundle is covered by a conductive material to bleed the charge to ground during operation of the gun. The grounding of the charge tubes enhances the charging of the powder and promotes safety by preventing the gun from storing a capacitive charge which could shock an operator or produce a spark, causing a fire or explosion.
One of the important factors in the magnitude of the charge imparted to the powder is the velocity of the powder through the gun; the higher the velocity of the powder, the higher the charge on the powder. Therefore, the powder is caused to flow through the gun at a high velocity in order to increase the charge on the powder. However, the velocity of the powder also has a detrimental effect on the wear life of the powder gun parts. Wear of the parts is also a function of velocity; the higher the velocity, the higher the wear. The powder abrades through the walls of the charge tubes in the charging portion of the gun with the result that the entire gun must be periodically returned to the manufacturer for rebuilding, at which time it is replaced by an entirely new or rebuilt gun.
Another important element in the performance of tribo-electric powder spray guns is the electrostatic grounding of the gun. Grounding of the prior art gun shown in U.S. Pat. No. 4,399,945 involves a very time-consuming and complicated manufacturing process. The charging tubes are preformed into convoluted shapes by heating them in special molds. The tubes were then arranged around an aluminum core and sprayed with a black graphite type conductive coating. A conductive wrapping is then applied around the entire tube bundle. A ground wire is extended from the core to the control panel for the unit.